The overall objective of the applicant's research is to identify new strategies for treating cancer which through selectivity will combine effectiveness with low toxicity. Because growth of new blood vessels is critical for tumor growth, angiogenesis is an attractive target for cancer therapy. The applicant has previously shown that Vascular Endothelial Growth Factor (VEGF), a cytokine of central importance for tumor angiogenesis, induces endothelial cell expression of the integrins alpha1beta1 and alpha2beta1 while the expression of several other integrins is either reduced or unchanged. In order to determine the functional significance of integrin expression in VEGF-induced angiogenesis, the applicant went on to show that alpha1 and alpha2 blocking antibodies in combination substantially reduced neovascularization in a murine tumor model. These data provide a foundation for the applicant's hypothesis that induction of alpha1beta1 and alpha2beta1 is a fundamental mechanism by which VEGF promotes neovascularization. Others initially demonstrated the feasibility of blocking angiogenesis with Abs to avb3 and avb5. The applicant finds that VEGF induces EC expression of the avb3 integrin (3-4 fold) indicating that induction of avb3 is another mechanism by which VEGF promotes angiogenesis. Collectively, these observations suggest the additional hypothesis that combinations of antagonists which block a1b1, a2b1, and av integrins may offer synergistic potency leading to greater selectivity towards angiogenesis and reduced toxicity towards other biological processes. The applicant proposes two Specific Aims. Specific Aim 1 is designed to investigate inhibition of angiogenesis and tumor growth in vivo by Abs which individually block a1b1 and a2b1 integrins and combinations of Abs which together block a1b1, a2b1, and av integrins. The applicant plans to employ the a1 and a2 antibodies separately to ascertain the consequences of blocking a1b1 and a2b1 individually, in comparison with blocking both simultaneously. In addition, the applicant plans complementary studies with a1 null mice to determine if antagonism of VEGF-driven angiogenesis by a1 Ab correlates with angiogenesis in the absence of a1 expression. The most significant findings will be tested in a mouse tumor model in order to determine if therapies involving a1 and/or a2 blocking Abs inhibit tumor growth. Specific Aim 2 is directed at investigating mechanisms by which the a1b1 and a2b1 integrins together with VEGF regulate microvascular EC functions in vitro. Independently, these integrins and VEGF have been implicated in the regulation of cell proliferation, cell migration, and collagenase (MMP-1) expression. This suggests the hypothesis that a1b1, a2b1 and VEGF cooperate to promote these functions which are all critical to angiogenesis. To test this hypothesis, the applicant plans to investigate cooperativity between VEGF and a1b1/a2b1 in promoting 1) activation of cyclin E-CDK2 and EC proliferation, 2) EC migration, and 3) induction of collagenase expression. It is expected that the proposed experimental plan will provide new and fundamental information on the mechanisms by which VEGF promotes angiogenesis and will provide important evidence in favor of anti-cancer therapies based on antagonism of a1b1 and/or a2b1.